The present invention relates to the medical diagnostic arts, It finds particular application in connection with dissipation of heat from a rotating anode of an x-ray tube for use with CT scanners and will be described with particular reference thereto. It should be appreciated, however, that the invention is also applicable to dissipation of heat in other vacuum systems.
A high power x-ray tube typically includes a thermionic filament cathode and an anode which are encased in an evacuated envelope. A heating current, commonly of the order of 2-5 amps, is applied through the filament to create a surrounding electron cloud. A high potential, of the order of 100-200 kilovolts, is applied between the filament cathode and the anode to accelerate the electrons from the cloud towards an anode target area. The electron beam impinges on a small area of the anode, or target area, with sufficient energy to generate x-rays. The acceleration of electrons causes a tube or anode current of the order of 5-200 milliamps. Only a small fraction of the energy of the electron beam is converted into x-rays, the majority of the energy being converted to heat which heats the anode white hot.
In high energy tubes, the anode rotates at high speeds during x-ray generation to spread the heat energy over a large area and inhibit the target area from overheating. The cathode and the envelope remain stationary. Due to the rotation of the anode, the electron beam does not dwell on the small impingement spot of the anode long enough to cause thermal deformation. The diameter of the anode is sufficiently large that in one rotation of the anode, each spot on the anode that was heated by the electron beam has substantially cooled before returning to be reheated by the electron beam.
The anode is typically rotated by an induction motor. The induction motor includes driving coils, which are placed outside the glass envelope, and an armature, within the envelope, which is connected to the anode. When the motor is energized, the driving coils induce electric currents and magnetic fields in the armature which cause the armature to rotate.
The temperature of the anode can be as high as 1,400.degree. C. Part of the heat is transferred to the armature and associated bearings. Most of the heat from the anode and armature is dissipated by thermal irradiation through the vacuum to the exterior of the envelope. Limited amounts of heat pass by conduction through the bearings and the bearing races. It is to be appreciated that heat transfer from the anode through the vacuum is limited. Overheating can cause damage to the anode, armature, and bearings, resulting in wobble and a lack of focus of the x-ray beam.
Several methods have been used to increase the rate of dissipation of heat from the rotor. In one method, a coating of chromium oxide or a mixed alumina-titanium oxide is applied to the armature. For high-end CT tubes, however, the coating is not fully effective at dissipating heat. The emissivities of chromium oxide and aluminum-titanium oxide are relatively low and the coating has adhesion problems, i.e. it tends to peel from the rotor with extended use.
In a second method, both the anode and vacuum envelope are rotated, while the cathode remains stationary. This configuration permits a coolant fluid to be circulated through the anode to provide a direct thermal connection between the anode and the exterior of the envelope. See, for example, U.S. Pat. Nos. 5,046,186; 4,788,705; 4,878,235; and 2,111,412.
One of the difficulties with this configuration is holding the cathode stationary within the rotating envelope. When the cathode assembly is supported by structures which are rotating with the envelope at a high speed, it tends to rotate with the anode and the envelope. Also, larger, more powerful motors are needed to rotate the larger anode and vacuum envelope assembly.
The present invention provides a new and improved x-ray tube and high emissivity coating which overcomes the above referenced problems and others.